CARLSBAD, N.M. (AP) - New air testing in the nation’s only underground nuclear repository showed no detectable radioactive contamination from a leak last month, the U.S. Department of Energy said Sunday.

Officials at the Waste Isolation Pilot Plant near Carlsbad said in a statement that the results came in as four more employees tested positive for low levels of radiation. The Energy Department earlier reported that 13 other workers were exposed, but they say all 17 aren’t likely to face any serious health effects and that there appears to be no danger aboveground.

Investigators sent instruments used to measure air quality and radioactivity underground Friday and Saturday in the first step toward resuming operations at the plant, which has been shut down since early February when a truck caught fire in a separate incident.

Air tests were done on a shaft and elevator system used to take personnel underground as well as the shaft that provides ventilation below the surface. Initial results indicate no contamination in the air or on the measuring equipment, officials said.

Meanwhile, federal officials say four additional workers now have tested positive for low-level radiation because the results took several weeks to come back. Those employees were part of the first crew to come on shift after the leak on Feb. 14. Thirteen employees working at the time of the leak also tested positive for contamination.

Donavan Mager, a spokesman for Nuclear Waste Partnership LLC, which runs WIPP, said urine samples for all 17 tested negative, indicating none of them actually inhaled any radioactive particles.

“That’s a good sign,” Mager said. “We don’t want to diminish the impact of employee health, but again, it’s a very positive sign that there will be no lasting health effects.”

Mager said employees have been able to work without having to wear protective equipment in the weeks since the leak.

WIPP officials say they may send personnel into the mine by the end of the week. Jose Franco, manager of the DOE’s Carlsbad Field Office, said days ago that the timing of cleanup, and the resources and costs involved, won’t be known until crews can actually enter.

Sensors alerted officials to a release of radiation on Feb. 14. Monitors as far as half a mile away later detected elevated levels of plutonium and americium in the air.

WIPP is the nation’s first underground nuclear repository and the only facility in the country that can store plutonium-contaminated clothing and tools from Los Alamos National Laboratory and other federal nuclear sites.